Check your page outs in the memory pane of activity monitor. If the number is big, you need to install more ram as you're running low on memory and this is causing a lot of unnecessary writes to your drive.

Ssd drives depend on garbage collection routines (trim, etc.) to erase the flash memory in unused blocks. This happens at idle. If enough free blocks are not erased, it will dramatically slow down write times.

So it really depends on your work flow as to how much free space is enough. If you're regularly working with large files like video, 10GB is probably not enough. If your usage is simple documents like word processing files and spreadsheets, 10GB will be plenty to support these and the various system overhead such as cache files.

I don't understand this "leave 10%" rule of thumb. Has it been factually proven with the SSD? Specifically the PCIe SSD in 2013 Airs? Or is it just a myth that has grown to legend?

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The 10% is not a hard and fast rule, but rather a general estimate. Performance can be reduced if a drive is too full, as there needs to be free space for growing logs, caches, user files, paging, etc. Obviously, if the drive is larger, such as 1TB, the percentage can be somewhat smaller. The point is, it's not a good idea to fill up the drive.

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